Adds a basemap layer at the very bottom of the render stack.
A basemap must never cover the data layers, so it is inserted before the first
existing layer. When the style carries no layer yet, addLayer without a
beforeId already appends at the bottom — hence the single-argument branch,
which callers can observe through the call arity.
Touches the map (style read + addLayer) but keeps no module state.
Adds a basemap layer at the very bottom of the render stack.
A basemap must never cover the data layers, so it is inserted before the first existing layer. When the style carries no layer yet,
addLayerwithout abeforeIdalready appends at the bottom — hence the single-argument branch, which callers can observe through the call arity.Touches the map (style read +
addLayer) but keeps no module state.